This invention relates to the field of asynchronous data communications, and more particularly, this invention relates to the field of decoding asynchronous digital data that is sent over a wire line, radio or fiber optic communication channel.
In the copending patent application Ser. No. 09/169,517, the problems of asynchronous communication systems and prior art methods for decoding were addressed. In that apparatus, the amplitude of a signal bit having negative and positive peak values was measured and a mid-bit reference computed. Changes in the base band signal peak excursions were tracked and the mid-bit reference was updated from the tracked changes of the base band signal excursions. To track thermal drifts and components aging, the positive and negative peak registers were periodically decremented and incremented, respectfully, at a rate several orders of magnitude slower than the ADC sample rate.
This apparatus addressed asynchronous communication systems that were commonly used where data was not time oriented and was sent in bursts and typically received in a burst receiver.
It is well known that in order to decode incoming data correctly, knowledge of the data amplitude should be ascertained in order to establish a reference within a communications receiver. Any receiver also should typically be capable of performing DC signal restoration during potentially long xe2x80x9cquietxe2x80x9d periods. An example of a xe2x80x9cquietxe2x80x9d period is shown in FIG. 1, where a frame of data has a stop bit, intermediate data (represented by the letter D) and a stop bit, followed by a two minute signal delay, followed by another frame of data with a start and stop bit and intermediate data (D).
Asynchronous digital data is often sent over a wire line, radio link, or fiber optic communication channel. In some prior art systems, analog implementations have been used for decoding the asynchronous signals. Typically, decoding circuits have been designed to take into consideration the length of a communication line or the link loss. In this type of system, the circuit will restore an analog signal to obtain a digital signal value and then make a decoding decision on the digital signal value. Typically, the circuit is designed based on knowledge of the difference in data amplitude coming into a communications receiver, such as in a fiber optic communications system.
In some prior art applications where the link distance is still known with certainty, the receiver could move even if the transmission distance and theoretical amount of attenuation is known. The link loss, however, would not be known and the signal strength could be variable. In some prior art asynchronous communication systems, designers typically would know the distance between a transmitter and receiver. For example, the communication line could be from room-to-room, or from city-to-city. Once this link distance is known, then the circuit is designed based on the theoretical amount of signal attenuation. There are often times when the amount of signal attenuation cannot be known in a communication line. Also DC signal restoration is difficult.
One of the key aspects of asynchronous data communication is the transmittal of packets or frames. Typically, a burst receiver may be used as mentioned before. In many prior art burst receivers, the receiver will lose its DC reference over a time pause in signal communication, which is why many communication systems use Manchester encoding or pseudo random number generation.
An example of one problem with DC signal restoration is comparing the difference between transmitting asynchronous data over three feet of cable between two computers in the same room, as compared to a longer distance system. In the asynchronous data communication system, if a logical 1 corresponds to five volts and a logical 0 corresponds to zero volts, there would be little problem in finding the midpoint reference or threshold when the data signal is transmitted through only three to five feet of cable. However, if the data signal is transmitted through 100 miles of cable, then the final attenuation would be great and the final signal could be about 100 millivolts. If the ideal threshold was set when only the three foot cable were used (i.e., a threshold of 2.5 volts), that threshold would be insufficient for the 100 mile transmission length and the threshold value would have to be lowered to about 50 millivolts. Thus, it is evident why the threshold in the most basic prior art asynchronous data communication system was set based on the transmission distance.
Some analog devices that are used for measuring the signal attenuation and setting a signal threshold in a receiver have been used with asynchronous data communications, but these systems do not accomplish high dynamic range burst mode asynchronous data decisions because of thermal drifts and parasitic effects of passive and active components.
Other common prior art methods have also been used to establish a signal reference in asynchronous data communication systems.
In a preamble system, several disposable data bits are sent prior to a data payload having the start bit to allow a decision circuit in the communications receiver to establish a signal reference. Thus, as soon as a start bit is transmitted, the circuitry has measured both zero and one logic levels and has set a threshold half-way between the zero and one logic levels.
In a second method known as xe2x80x9cavoidance of risk mode of operation,xe2x80x9d a continuous stream of data is sent with an even mix of ones and zeros by using a randomizing circuit. To avoid a long quiet period, which sometimes occurs such as a two minute delay as shown in FIG. 1, the data is multiplied by a pseudo random number in the transmitter to ensure the data stream is rich in data transitions. Thus, there will always be ones and zeros that are transmitted. Then the communications receiver circuitry decodes the pseudo random number and extracts the information.
In another method known as automatic gain control (AGC), a constant automatic gain control circuit controls the communications receiver gain. For example, if the transmission line is 100 miles long and there is a 100 millivolt threshold, then the signal will be amplified back to five volts. As a result, the system would use a 2.5 volt reference. However, there are time constants. If the system is quiet for extended periods, for example, about two minutes or some other time period, then the signal will decay and there is no signal information on which to make a gain or adjustment of the signal. Thus, the DC signal information is not present.
It is evident that the prior art methods are not adequate in some circumstances. For example, the preamble system reduces the data rate of the channel. As a result, in asynchronous data transmission, the preamble method prevents binary data transfer. Certain characters could be sent as a preamble and would, therefore, not be used for the information content. For example, if bits are sent to a communications receiver in order to synchronize that receiver, then no data information is sent. Thus, the preamble system requires a preamble and prevents binary data transfer. If binary data is transmitted, there could be no start bit. The preamble system would not know the difference between a binary data bit or preamble bit. There could also never be a xe2x80x9csyncxe2x80x9d word in the binary transmission because it would mistake it for a frame signal. In the xe2x80x9cavoidance of the first mode of operation,xe2x80x9d as noted before, a continuous transmission is required for randomized data. The automatic gain control system (AGC) solves the problem of amplitude fluctuations, but does not solve the problem of DC restoration. In an automatic gain control system, it is possible to correct the gain, but it cannot affect the DC reference point.
Although the apparatus disclosed in copending U.S. patent application Ser. No. 09/169,517 discloses a method and apparatus that decodes asynchronous data signals, that apparatus has not been found feasible with a signal having significant distortions, amplitude variations and DC offsets over a wide dynamic range, such as greater than 40 dB.
It is therefore an object of the present invention to provide an apparatus and method for decoding asynchronous data signals where the communication link contains unanticipated signal distortions that prevent reliable communication.
In accordance with the present invention, the apparatus digitally decodes asynchronous communication signals and includes a sampling and storing circuit for sampling a digitally converted asynchronous communication signal and storing sampled signal values corresponding to negative edge signal values, positive edge signal values and central signal values. A derivative calculating circuit is connected to the sampling and storing circuit for receiving at least negative edge signal values and positive edge signal values and calculating derivatives. A comparator circuit receives the calculated derivatives and compares the derivatives with a threshold based on a calculated negative derivative and produces negative output and positive output values used for determining a decoded alpha signal.
The derivative calculating circuit includes a plurality of subtraction circuits that receive sampled signal values to calculate a plurality of derivatives. A threshold control circuit is connected to the derivative calculating circuit and receives a derivative calculated from a maximum negative and positive edge signal value. A bit timer receives the negative output and positive output values and times operation of the sampling and storing circuit, derivative calculating circuit and comparator circuit. An automatic gain control circuit is connected to the threshold circuit for receiving compared values of previously stored and decoded output signals. A storage register stores samples of the decoded output signal and an output control circuit receives negative output and positive output values and produces decoded output signals.
A method of the present invention is also disclosed for digitally decoding asynchronous communication signals. A digitally converted asynchronous communication signal is sampled and a plurality of sampled signal values are stored corresponding to negative edge signal values, positive edge signal values and central signal values. The method further comprises the step of calculating derivatives based on sampled signal values and comparing the calculated derivatives for the threshold based on the calculated negative derivative. Negative output and positive output values are produced.